Typical commercially available vehicle transmission fluid coolers are simply a plate type heat exchanger that is placed in one tank of the engine cooling radiator. External lines run back and forth from the transmission case to take hot transmission fluid to and from the radiator tank and the transmission oil heat exchanger. A separate coolant pump, typically called a water pump, circulates coolant through the engine block and the radiator to cool the engine. The radiator tank is used as a convenient heat sink, but that is the sole extent of the operational or structural cooperation between the engine and transmission cooling systems. Therefore, during engine warm-up, when the typical radiator thermostat is closed and coolant is not being pumped through the radiator (so as to allow the engine to warm up), transmission heat is simply statically dumped to a cold radiator tank.
Designs have been proposed, at least on paper, to structurally integrate engine oil coolers (but not transmission fluid coolers) with vehicle engine coolant pumps and systems. U.S. Pat. No. 4,370,957 to Skatsche shows a housing in which a centrifugal impeller 6 sends coolant to and through an adjacent oil cooler 10'. After flowing through and around the oil cooler 10', and picking up heat from the engine oil that is concurrently circulating therethrough, the engine coolant is sent out to a non-illustrated conventional engine block and, presumably, a radiator. While lengthy engine block to oil cooler lines are avoided by bolting the assembly directly to the engine block, a great drawback is that the oil cooler presents a constant, in series flow restriction to the engine coolant pump. Coolant cannot flow to the engine and radiator without passing through the restrictive oil cooler heat exchanger first. Another design, shown in U.S. Pat. No. 4,520,767 to Roettgen et al does not so much combine the oil cooler into the coolant pump structurally as it combines the engine and oil cooling systems, as well as an aftercooler. Two separate cooling "loops" are created, one for the engine, and one for the oil cooler, radiator and aftercooler. Coolant from the pump 40 is always directed first to and through the oil cooler 10. From oil cooler 10, coolant may, depending on the temperature dependent position of a so called bypass valve 52, flow through the radiator 16 and the aftercooler 20, or just through the aftercooler 20 (FIG. 3). This is a very complex system practically suitable only for large trucks, and suffers from the same drawback as the Skatsche design, in that coolant flowing to the radiator 30 from the pump 40 must first pass through the oil cooler 10. Despite the "by pass" valve, coolant from the pump never really bypasses the in series flow restriction created by the oil cooler 10.